It takes 114 strokes of front crawl to swim from one end to the other. Through tinted goggles, the intense blueness seems more subtropical than south London, but, pausing for breath at the shallow end, beyond the roar of the fountain, one can just make out the rumble of the London-to-Brighton line.
Tooting Bec Lido, at 90 metres long and 30 metres wide, is the biggest outdoor pool in Bri tain, supposedly the second largest in Europe, and one of London's summer treasures. On one's arrival, the scene never fails to raise lift the spirits - the huge rectangle of blue, the impassive lifeguards gazing down from their high chairs, the bright red, yellow and green changing huts along either side of the pool. Below the waterline you enter another world, seemingly without end, uninterrupted by lanes or tiles, with only two black stripes giving the game away. As you move into deeper water, the bottom of the pool becomes speckled with dust and sand like the seabed, and far away you glimpse a school of giant squid and a seahorse, which in time morph into a group of teenagers clinging to the wall of the deep end.
The unheated water takes a bit of getting used to, but its uncompromising chill is part of the attraction. On a hot day, there is little better than stretching out beside this Victorian behemoth with a novel and a picnic, once in a while submitting oneself to the exquisite shock of immersion.
While the essence of indoor pools is chlorinated enclosure, lidos are all about escapism. Their heyday was between the wars, a time when the competing diversions of colour television, package holidays and Fitness First were yet to be discovered. Open-air swimming became a weapon in improving public health and a sign of municipal pride, and nowhere was it more enthusiastically pursued than London. The story is set out magisterially in Janet Smith's 2005 book Liquid Assets. Operating between 1889 and 1965, the London County Council - predecessor to the Greater London Council - oversaw the development of 60 open-air pools by the early 1950s. One of the main instigators was Herbert Morrison, LCC chairman between 1934 and 1940, who promised to turn London into a "city of lidos" where no one would have to travel more than a mile and a half to find his or her nearest pool.
Although the dream was never realised, his legacy is still impressive. The big cities of northern England can no longer count a single lido between them, but London, despite regular closures in the 1970s and 1980s, has nine, in add ition to the ponds of Hampstead Heath and the Serpentine. With one derelict lido having reopened last autumn in the East End of London and another returning two years from now to Uxbridge, in the west, the tide appears to be turning in favour of open-air pools. On 2 July, Brockwell Park Lido also reopened its doors after a major refit to celebrate its 70th birthday.
London Fields is the other new kid on the block, enjoying its first summer season after 20 years lying derelict. It is an unprecedented comeback and Hackney Council is to be praised for rewriting the script on open-air pools. But although a unique addition (it will be London's only heated 50-metre lido), the pool is something of a dis appointment. All automatic doors, sterile reception, officious signage and safety bars, and rather narrow in size, it evokes the leisure centre rather than the lido. The ethos is summed up by a notice on its website warning that picnics are prohibited. Perhaps it will come into its own in the winter, though. Much-vaunted plans for an inflatable bubble to enclose the pool seem to have been dropped. Instead, a blue tarpaulin roof will be unwound overhead, and thus the vital feeling of being outdoors will be preserved. On a dark December night, with a gale ripping across east London from Siberia, entering the heated water will seem both cosy and intrepid.
When it comes to design, most commentators revere the modernist lines of Saltdean Lido, on the outskirts of Brighton, but really this is one for the photographers. In the flesh, as it were, the building is rather banal and the pool, now unpardonably sliced into two, appears puny. For my money, it is hard to beat the geometrical solemnity of the 60-metre Parliament Hill Fields Lido by Hampstead Heath. It is technologically unique, having been fitted with a new steel lining in 2005, and the water has a silver-green hue that responds intriguingly to the light as you swim up and down. It's where Alastair Campbell trained for his triathlon, though the day I visited the only whiff of new Labour was the potential threat of Asbos for a group of exuberant teenagers.
Brockwell Park Lido opened a year before Parliament Hill Fields, and has a similarly symmetrical design. If Tooting is one for the hearties - impressive, but rather puritanical - Brockwell, with its Rastafarian announcer on the public address system, its lush Virginia creeper, its well-stocked bar and its tolerance of smoking in various forms, was the pool with the most hedonistic and socially mixed crowd.
I hope that the atmosphere survives the make over, if not the smoking ban. The pool reopened on the Monday at 6.45am sharp after the refurbishment and redevelopment, commissioned in 2003 and carried out by the architects Pollard Thomas Edwards. The idea is to preserve the listed building but provide the lido with a steady income stream all year round from yoga lessons, t'ai chi and children's music and drama classes.
At the pool's 70th birthday party, I will be raising a glass to Herbert Morrison. One wonders if, in 2077, when foreign holidays, private pools and air conditioning will be carbon crimes, Londoners will be toasting Sir Ken Livingstone for his swimming legacy. So far, preparations for the 2012 Olympics seem to be doing more harm than good to London's swimming facilities. What most of us crave is not a competition-standard pool, but somewhere local, with its own distinct identity and enough room to swim at your own pace, where you can cool off on a hot day. How about a new lido, Ken?